3,238 research outputs found
Towards the application of the Maximum Entropy Method to finite temperature Upsilon Spectroscopy
According to the Narnhofer Thirring Theorem interacting systems at finite
temperature cannot be described by particles with a sharp dispersion law. It is
therefore mandatory to develop new methods to extract particle masses at finite
temperature. The Maximum Entropy method offers a path to obtain the spectral
function of a particle correlation function directly. We have implemented the
method and tested it with zero temperature Upsilon correlation functions
obtained from an NRQCD simulation. Results for different smearing functions are
discussed.Comment: Lattice 2000 (Finite Temperature
Quantization and Periodicity of the Axion Action in Topological Insulators
The Lagrangian describing the bulk electromagnetic response of a
three-dimensional strong topological insulator contains a topological `axion'
term of the form '\theta E dot B'. It is often stated (without proof) that the
corresponding action is quantized on periodic space-time and therefore
invariant under '\theta -> \theta +2\pi'. Here we provide a simple, physically
motivated proof of the axion action quantization on the periodic space-time,
assuming only that the vector potential is consistent with single-valuedness of
the electron wavefunctions in the underlying insulator.Comment: 4 pages, 1 figure, version2 (section on axion action quantization of
non-periodic systems added
Dynamical invariants for quantum control of four-level systems
We present a Lie-algebraic classification and detailed construction of the
dynamical invariants, also known as Lewis-Riesenfeld invariants, of the
four-level systems including two-qubit systems which are most relevant and
sufficiently general for quantum control and computation. These invariants not
only solve the time-dependent Schr\"odinger equation of four-level systems
exactly but also enable the control, and hence quantum computation based on
which, of four-level systems fast and beyond adiabatic regimes.Comment: 11 pages, 5 table
Coreless and singular vortex lattices in rotating spinor Bose-Einstein condensates
We theoretically investigate vortex-lattice phases of rotating spinor
Bose-Einstein condensates (BEC) with the ferromagnetic spin-interaction by
numerically solving the Gross-Pitaevskii equation. The spinor BEC under slow
rotation can sustain a rich variety of exotic vortices due to the
multi-component order parameters, such as the Mermin-Ho and Anderson-Toulouse
coreless vortices (the 2-dimensional skyrmion and meron) and the
non-axisymmetric vortices with the sifting vortex cores. Here, we present the
spin texture of various vortex-lattice states at higher rotation rates and in
the presence of the external magnetic field. In addition, the vortex phase
diagram is constructed in the plane by the total magnetization and the
external rotation frequency by comparing the free energies of possible
vortices. It is shown that the vortex phase diagram in a - plane may
be divided into two categories; (i) the coreless vortex lattice formed by the
several types of Mermin-Ho vortices and (ii) the vortex lattice filling in the
cores with the pure polar (antiferromagnetic) state. In particular, it is found
that the type-(ii) state forms the composite lattices of coreless and
polar-core vortices. The difference between the type-(i) and type-(ii) results
from the existence of the singularity of the spin textures, which may be
experimentally confirmed by the spin imaging within polarized light recently
proposed by Carusotto and Mueller. We also discussed on the stability of
triangular and square lattice states for rapidly rotating condensates.Comment: to be published in Phys. Rev.
Dynamically stable multiply quantized vortices in dilute Bose-Einstein condensates
Multiquantum vortices in dilute atomic Bose-Einstein condensates confined in
long cigar-shaped traps are known to be both energetically and dynamically
unstable. They tend to split into single-quantum vortices even in the ultralow
temperature limit with vanishingly weak dissipation, which has also been
confirmed in the recent experiments [Y. Shin et al., Phys. Rev. Lett. 93,
160406 (2004)] utilizing the so-called topological phase engineering method to
create multiquantum vortices. We study the stability properties of multiquantum
vortices in different trap geometries by solving the Bogoliubov excitation
spectra for such states. We find that there are regions in the trap asymmetry
and condensate interaction strength plane in which the splitting instability of
multiquantum vortices is suppressed, and hence they are dynamically stable. For
example, the doubly quantized vortex can be made dynamically stable even in
spherical traps within a wide range of interaction strength values. We expect
that this suppression of vortex-splitting instability can be experimentally
verified.Comment: 5 pages, 6 figure
Experimental determination of the Berry phase in a superconducting charge pump
We present the first measurements of the Berry phase in a superconducting
Cooper pair pump. A fixed amount of Berry phase is accumulated to the
quantum-mechanical ground state in each adiabatic pumping cycle, which is
determined by measuring the charge passing through the device. The dynamic and
geometric phases are identified and measured quantitatively from their
different response when pumping in opposite directions. Our observations, in
particular, the dependencies of the dynamic and geometric effects on the
superconducting phase bias across the pump, agree with the basic theoretical
model of coherent Cooper pair pumping.Comment: 4 pages, 3 figure
Minimal and Robust Composite Two-Qubit Gates with Ising-Type Interaction
We construct a minimal robust controlled-NOT gate with an Ising-type
interaction by which elementary two-qubit gates are implemented. It is robust
against inaccuracy of the coupling strength and the obtained quantum circuits
are constructed with the minimal number (N=3) of elementary two-qubit gates and
several one-qubit gates. It is noteworthy that all the robust circuits can be
mapped to one-qubit circuits robust against a pulse length error. We also prove
that a minimal robust SWAP gate cannot be constructed with N=3, but requires
N=6 elementary two-qubit gates.Comment: 7 pages, 2 figure
Translations and dynamics
We analyze the role played by local translational symmetry in the context of
gauge theories of fundamental interactions. Translational connections and
fields are introduced, with special attention being paid to their universal
coupling to other variables, as well as to their contributions to field
equations and to conserved quantities.Comment: 22 Revtex pages, no figures. Published version with minor correction
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